Arrangement for monitoring at least one parameter for a number of motor vehicle wheels

ABSTRACT

The arrangement for monitoring at least one parameter for a number of motor vehicle wheels uses at least one receive antenna that features a directional characteristic such that at least the signals which are transmitted by detector units located at two different vehicle wheels can be uniquely assigned to the relevant wheel positions as a result of the different receive level. The use of a number of receive antennas allows either the number of distinguishable detector units to be increased or a redundancy to be created to improve the reliability of the detection.

PRIORITY

[0001] This application claims foreign priority of the Germanapplication DE 102 30 098.4 filed on Jul. 4, 2002.

TECHNICAL FIELD OF THE INVENTION

[0002] The invention relates to an arrangement for monitoring at leastone parameter for a number of motor vehicle wheels.

BACKGROUND OF THE INVENTION

[0003] Monitoring parameters of a motor vehicle wheel, for example thetire pressure or the tire temperature, plays a decisive role in thesafety of the vehicle or the driver. To dispense with the need formanual checking of these types of parameters devices have been developedwith which it is possible to record the parameters of the wheelsautomatically and display them using the appropriate display devices onthe dashboard for example. In such cases it is desirable not only todisplay the parameters for all the cases in which the allowable range ofa parameter has been exceeded but also to specify a unique assignmentbetween the displayed parameters and the position of the wheelconcerned. This necessitates an arrangement of this type to be designedin such a way that this assignment will be correctly made even after awheel has been changed.

[0004] As part of an assignment or initialization process, which isundertaken as required or at predetermined intervals, it is necessary toassign the identifier of each detector unit which is allocated to awheel to the appropriate wheel position, for example “front left”,“front right”, “rear right”, “rear left”. In normal operating mode, theidentifier, which is contained in a signal sent by one of the number ofdetector units can then be used as a basis for assigning the relevantwheel position, by the detected identifier being compared with thestored assignment information (the identifiers assigned to wheelpositions are stored).

[0005] It would of course also be conceivable to execute this type ofassignment process each time before a signal of a specific detector unitis interpreted. As a rule however this is too awkward andtime-consuming.

[0006] A method of assigning transmitters to receive antennas in tirepressure monitoring systems is known from EP-B-0 861 160, in which apressure measurement sensor, a transmitter and a send antenna are eachassigned to a wheel. In addition each wheel is assigned a receiveantenna on the bodywork which is connected via a cable in each case tothe receiving and evaluation electronics. The assignment of theidentifiers to wheel positions is made by a signal sent from atransmitter or from the associated transmit antenna being received byall receive antennas and that the (wheel) position of the receiveantenna which delivers the signal with the greatest intensity isassigned to the corresponding transmitter and its identifier.

[0007] The disadvantage here is that each wheel position must beassigned a receive antenna, which in its turn must be connected via acorresponding cable to the receiving and evaluation electronics. Thisbrings corresponding installation effort and correspondingly high costsfor the number of receive antennas corresponding to the number ofwheels.

[0008] A system for recording tire pressures is known from U.S. Pat. No.5,774,047, in which a detector unit with a transmitter is also assignedto each wheel. The transmit signals are evaluated and assigned to thewheel positions by providing at least two receive antennas, whereby thephase difference and polarity of the two receive signals in each case isevaluated, which is delivered by the at least two receive antennas onreceiving the signal of one and the same transmitter.

[0009] This type of evaluation of the polarity and phase relationshipsof the signals is however linked to considerable expense as regards thecircuitry.

SUMMARY OF THE INVENTION

[0010] The underlying object of the invention is thus to create anarrangement for monitoring at least one parameter from a number ofvehicle wheels, in which a receive signal which is sent by one or moredetector units can be simply assigned to the corresponding wheelposition with little effort.

[0011] The invention may achieve this object by an arrangement tomonitor at least one parameter for a number of motor vehicle wheels,comprising a number of detector units, each of which is assigned to awheel, whereby each detector unit comprises a sensor for recording theat least one parameter and a transmit unit with a transmit antenna, atleast one receive antenna comprising a directional characteristicpositioned in such a way in the vehicle that, in connection with thefield strengths of the signals sent by the individual transmit units atthe location of the at least one receive antenna produces receivesignals of different power in each case for at least two of the transmitunits, a central evaluation unit which comprises a receive unit, whereinthe receive unit is connected to the at least one receive antenna; andan evaluation unit coupled with the central evaluation unit whichcompares the receive power of a received signals with stored thresholdvalues or ranges of values assigned to one wheel position in each caseand assigns the received signal to a wheel position, if the thresholdvalue assigned to this wheel position is exceeded or the receive powerlies within the range of values assigned to this wheel position.

[0012] The directional characteristic of the at least one receiveantenna may be created such that, starting from the position of thereceive antenna, they each produce sensitivities in the direction of thetransmit antennas of the at least two signals to be distinguished of therelevant transmit units, for which the difference is greater than apredefined value, in which case this value is selected to enable a cleardistinction to be made between the signals using the detected receivepower of the two signals. For a vehicle with four wheels two receiveantennas may be provided. One receive antenna may be provided in aposition in the area of the front wheel and one receive antenna may beprovided in a position in the area of the rear wheel, preferably in thearea above the wheel arch. The directional characteristic of the receiveantenna at least in the direction of the further wheel of the samelengthwise side of the vehicle may show a low or high sensitivity and inthe direction of the further front or rear wheel a high or lowsensitivity, so that the evaluation unit can use one receive antenna ineach case to undertake at least one unique assignment of the signals ofthe transmit units at the wheel positions in the direction of the lowand high sensitivity. The directional characteristic of the receiveantennas may be created so that the signal of the transmit unit in thearea of the same wheel creates the highest receive power in each case,which is clearly distinguishably higher than the receive power onreception of a signal from the transmit unit in the direction of thehigh sensitivity of the directional characteristic. The transmitantennas or the entire transmit units or detector units may be identicalin design. In the receive unit there may be provision for a furtherreceive antenna and the receive unit may be positioned adjacent to atransmit unit in such a way that this receive antenna receives thesignal of the adjacent transmit unit with the greatest signal power. Theevaluation unit may undertake the assignment of the signals to wheelpositions on request or at regular intervals as part of an assignmentmode and when doing so may assign a characteristic identifier for thedetected wheel position transmitted by one of the transmit units in eachcase and may store it and wherein the evaluation unit in normaloperation assigns the received signals to the wheel positions using acomparison between the identifier transmitted by the transmit units andthe stored assignment information (identifier for wheel position). Thereceive unit may comprise a controllable switch that in each caseconnects one of a number of receive antennas with downstream componentsof receive unit.

[0013] The object can also be achieved by a method for monitoring atleast one parameter for a number of motor vehicle wheels, comprising thesteps of:

[0014] recording the at least one parameter;

[0015] transmitting the at least one parameter with a transmit antenna,

[0016] positioning of at least one receive antenna in such a way in thevehicle that, in connection with the field strengths of the signals sentby the individual transmit units at the location of the at least onereceive antenna produces receive signals of different power in each casefor at least two of the transmit units; and

[0017] comparing the receive power of a received signals with storedthreshold values or ranges of values assigned to one wheel position ineach case and assigning the received signal to a wheel position, if thethreshold value assigned to this wheel position is exceeded or thereceive power lies within the range of values assigned to this wheelposition.

[0018] The method may further comprise the step of starting from theposition of the receive antenna, distinguishing at least two signals ofthe relevant transmit units by producing sensitivities in the directionof each of the transmit antennas, for which the difference is greaterthan a predefined value, in which case this value is selected to enablea clear distinction to be made between the signals using the detectedreceive power of the two signals. The method may further comprise thestep of providing one receive antenna in a position in the area of afront wheel and one receive antenna in a position in the area of a rearwheel, preferably in the area above the wheel arch. The directionalcharacteristic of the receive antenna at least in the direction of thefurther wheel of the same lengthwise side of the vehicle may show a lowor high sensitivity and in the direction of the further front or rearwheel a high or low sensitivity, so that the evaluation unit can use onereceive antenna in each case to undertake at least one unique assignmentof the signals of the transmit units at the wheel positions in thedirection of the low and high sensitivity. The method may furthercomprise the step of creating the directional characteristic of thereceive antennas in such a way that the signal of the transmit unit inthe area of the same wheel creates the highest receive power in eachcase, which is clearly distinguishably higher than the receive power onreception of a signal from the transmit unit in the direction of thehigh sensitivity of the directional characteristic. The method mayfurther comprise the steps of providing for a further receive antenna inthe receive unit and positioning the receive unit adjacent to a transmitunit in such a way that this receive antenna receives the signal of theadjacent transmit unit with the greatest signal power. The evaluationunit may undertake the assignment of the signals to wheel positions onrequest or at regular intervals as part of an assignment mode and whendoing so may assign a characteristic identifier for the detected wheelposition transmitted by one of the transmit units in each case andstores it and the evaluation unit in normal operation may assign thereceived signals to the wheel positions using a comparison between theidentifier transmitted by the transmit units and the stored assignmentinformation (identifier for wheel position). The method may furthercomprise the step of connecting one of a number of receive antennas withdownstream components of the receive unit by means of a switch.

[0019] The starting point for the invention is the knowledge that theassignment of a receive signal to the wheel position (or vice-versa) canbe achieved in a simple manner by using at least one receive antennawhich features a directional characteristic such that signals sent inconjunction with the field strengths of the signals sent by theindividual transmitters at the location of at least one receive antennaproduce receive signals of different powers for at least two transmitunits to be distinguished. This means that it is sufficient to simplycompare each signal level with stored assignment information to allowthe received signal to be assigned to the corresponding wheel position.The stored assignment information comprises a threshold value assignedto each wheel position in each case or a range of values assigned to awheel position in each case for the receive signal power or the level(voltage) of the receive signal.

[0020] In this case the directional characteristic of the at least onereceive antenna can be designed in such a way that the receivecharacteristic in each direction, starting from the position of thereceiver antenna in the direction of the wheel concerned or the positionof the detector unit located on it shows a markedly differentsensitivity in each case. This would make it possible using a singleantenna, to distinguish between all signals of for example four detectorunits of the vehicle with four wheels.

[0021] According to the preferred form of embodiment of the invention,two receive antennas are provided for a vehicle with four wheels. Thepreferred configuration here is one receiver antenna in the area of onefront wheel and one receive antenna in the area of one rear wheel. Thedirectional characteristic of the two receive antennas can be designedso that in the direction of the other wheel on the same side of thevehicle there is a low (high) sensitivity and in the direction of theother front or rear wheel a high (low) sensitivity.

[0022] In this way the evaluation unit can use one antenna in each caseto at least make a unique assignment of the wheel positions to thesignals of the transmit units at the wheel positions in the direction ofthe low and high sensitivity.

[0023] With a further form of embodiment of the invention thedirectional characteristic of the receive antenna is further created insuch a way or the receive antenna aligned in relation to the wheelposition, in whose environment the receive antenna is arranged such thatthe signal of the detector unit assigned to the wheel at this wheelposition generates the highest receive power. This receive power shouldbe distinctly and identifiably higher than at the receive power onreceipt of a signal from the transmit unit in the direction of highsensitivity of the directional characteristic of the relevant receiveantenna.

[0024] The benefit produced in this way is that by using the one antennathe signals of three detector units can be uniquely assigned to thewheel positions concerned. This similarly applies to the other receiveantenna. In this case the redundancy provided (a number of wheelpositions can be assigned using both the one and the other receiveantenna) enables higher security to be guaranteed for the assignment.

[0025] With the arrangement in accordance with the invention thetransmit antennas of the detector units or the entire transmit units ordetector units can be designed identically. It is not necessary todesign the transmitter units in such a way that the ability todistinguish between the relevant transmit signals is made possible bydiffering transmit powers.

[0026] In the evaluation unit a further receive antenna can be providedwhich can also feature a defined directional characteristic in relationto the wheel positions. This additional receive antenna then serves as aredundant unit and can be included for checking and if necessarycorrecting the assignment results.

[0027] In accordance with the invention the assignment of the signals ofthe transmit units or detector units to the wheel positions can beundertaken as part of an initialization or assignment process on requestor at predetermined intervals. In this case an assignment of thecharacteristic identifiers sent by the transmit units in each case ismade to the wheel position concerned.

[0028] The assignment of the receive signals to wheel positions can ofcourse also be undertaken before or during each receive process. Thetransmission of an identifier can be dispensed with entirely ifnecessary in this case.

[0029] In accordance with the preferred form of embodiment of theinvention the central evaluation unit or the receive unit included in itfeatures a controllable switch which in each case connects one of thenumber of receive antennas to the downstream components of the receiveunit or the evaluation unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The invention is described in more detail below on the basis ofthe exemplary embodiments shown in the drawing. The drawings show

[0031]FIG. 1 a schematic diagram of a vehicle with a first form ofembodiment of an arrangement for monitoring at least one parameter ofthe four vehicle wheels;

[0032]FIG. 2 a schematic diagram of a vehicle with a second form ofembodiment of an arrangement for monitoring at least one parameter ofthe four vehicle wheels;

[0033]FIG. 3 a schematic diagram of the directional characteristic of asimple dipole-type receive antenna; and

[0034]FIG. 4 a simplified diagram of the directional characteristic ofan additional antenna to be positioned in the central evaluation unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The schematic diagram of a vehicle 1 in FIG. 1 shows the vehiclewith an arrangement 3 to monitor at least one parameter for a number (inthe example shown, four) of vehicle wheels. The arrangement 3 comprisesa total of four detector units 5, whereby one detector unit 5 isassigned to a vehicle wheel 7 in each case. Each detector unit 5includes a transmitter unit which has not shown in more detail, whichfor its part features a transmit antenna in each case. The detectorunits 5 are preferably designed to be identical for practical reasons.

[0036] Arrangement 3 further comprises two receive antennas 9, wherebyin the exemplary embodiment shown in FIG. 1 one receive antenna 9 ispositioned in the area of the front left vehicle wheel 7 and one receiveantenna 9 in the area of the rear left vehicle wheel 7 in a fixedposition. To aid clarity the diagram in FIG. 1 does not show the vehiclewheels on the left-hand side of the vehicle. Instead of the directionalcharacteristic of the receive antenna 9 are shown schematically.

[0037] Finally arrangement 3 includes a central evaluation unit 11 whichfeatures an evaluation unit 13 and its upstream receive unit 15.

[0038] The receive antennas 9 are connected to the receive unit 15, inwhich case the receive unit 15 features a controllable switch 17 whichdirects the signal of a receive antenna 9 in each case to the furthercomponents of the receive unit 15.

[0039] The relatively simple directional characteristic of receiveantenna 9 shown in FIG. 1 and in greater detail in FIG. 3 provide theopportunity, in conjunction with a selected position and alignment ofthe receive antennas nine, to distinguish between the transmit signalsof detector units 5 in the wheel positions A, B, C, D.

[0040] Given an identical design of detector units 5, the followingrelationships between the receive powers all signal levels P of thereceive signals for transmit signals of the detector units in the wheelpositions A, B, C, D will be produced for receive antenna and 9 in theassigned Position I which is adjacent to wheel position A:

[0041] P_(I)(A)>P_(I)(B)>P_(I)(C)>P_(I)(D)

[0042] For the receive antenna 9 in the antenna Position II, which isadjacent to wheel position D, the following relationships between thereceive signal levels P for transmit signals which are created by thedetector units 5 in the wheel positions A, B, C, D are produced in asimilar way:

[0043] P_(II)(D)>P_(II)(C)>P_(II)(B)>P_(II)(A)

[0044] Since the spacing of the vehicle wheels in the longitudinaldirection is as rule greater than the spacing of the wheels on a vehicleaxle, even with the selected receive characteristic the signals P_(I)(B)und P_(I)(C) or the receive signals P_(II)(C) and P_(II)(B) can still beidentified although the sensitivity of the simple dipole characteristicselected in the example in the chosen receive direction is stillrelatively similar.

[0045] Of course the directional characteristic of the receive antenna 9can be designed so that the sensitivity in these directions differsmarkedly, so that in this way it is possible to make an even more securedistinction of the receive signals of the detector units located inthese wheel positions.

[0046] A method is explained briefly below by which the centralevaluation unit 11 distinguishes and assigns the signals transmitted bythe individual detector units 5.

[0047] It should be pointed out first of all that this type ofdistinction or assignment of a received signal can of course beundertaken before or during each reception of a signal sent by adetector unit 5. Instead of this however the preferred method is to usean assuagement or initialization process that can be executed by thecentral evaluation unit 11 on request or at pre-specified intervals thatinvestigates each of the detector units 5 in turn to see the wheelposition A, B, C, D at which these are located and to store thisinformation assigned to a characteristic indentifier for the detectorunit concerned in the central evaluation and control unit 11. Aftertermination of the assignment process the assignment can then be made inthe normal operating mode of arrangement 3 in which the identifier of areceive signal is merely detected and compared with the storedassignment information. Normal operating mode does not require anyswitch-over of the receive antennas either. Instead it is sufficient todetect the receive antennas of all four detector units 5 with one andthe same receive antenna 9.

[0048] If an assignment of the receive signals to wheel positions is tobe undertaken—whether as part of an assignment or initialization processor on receipt of each signal in the normal operating mode—, switch 17 isfirst used to connect a receive antenna with the receive unit 15 and therelevant signal level is determined. In a next step switch 17 isconnected with the other receive antenna 9 and the level of the receivesignal as a result of receiving a this same signal with this receiveantenna 9 is determined. The evaluation unit 13 then investigates (oreach receive level after it is established), whether this receive levelexceeds a threshold that is assigned to the relevant wheel positions A,B, C, D or whether this level lies within range of values assigned inone of the relevant wheel positions. If for example the receive levelP_(I) of a signal which is delivered by a receive antenna 9 in positionI exceeds a threshold value which is assigned to wheel position B butnot the threshold value which is assigned to wheel position A, thereceive signal of wheel position B is assigned. Thus the process forassigning a signal in the normal operating phase is ended. If anassignment or initialization process is undertaken, the identifier ofthe signal concerned is detected and stored as assignment informationassigned to the relevant wheel position. Furthermore these steps areexecuted in an assignment or initialization process for the signals sentby all the detector units.

[0049] Since both receive antennas 9 at least theoretically allow aunique assignment of a signal as a result of the selected directionalcharacteristic, the evaluation units 13 cannot just undertake assignmentof the signal as a result of the level of receive antenna 9 in positionI, but also an assignment of the signal by investigating the levelP_(II) that is delivered by the receive antenna 9 in position II. Thisredundancy produces increased safety.

[0050] If the detected assignments of the receive signal for the wheelposition concerned differ as a result of the investigation of the leveldelivered by the two antennas, the result can be discarded and theprocess executed again.

[0051] In the same way the receive signals that are delivered by thereceive antennas 9 in position I or II are only used for the assignmentof specific wheel positions, for example those wheel positions that canbe distinguished and assigned with the greatest reliability.

[0052] The further exemplary embodiment shown in FIG. 2 differs from theconfiguration shown in FIG. 1 merely in that the receive antennas 9 onopposite lengthwise sides of the vehicle are positioned in the area ofthe front or rear wheel arch.

[0053] With this form of embodiment it is possible, as mentionedpreviously, to only use receive antenna 9 in position I for assigning areceive signal to the wheel positions B and C. As a result of the simpledirectional characteristic there is the danger that the signals of thedetector units in wheel positions A and D will only differ slightly intheir receive level. These wheel positions can however be securelyassigned by evaluating the receive signal supplied by receive antenna 9in position II. For the form of embodiment shown in FIG. 2 the followingequation for the relationships of the receive level is produced forreceive antenna 9 in position I:

[0054] P_(I)(B)>P_(I)(A)>P_(I)(D)>P_(I)(C)

[0055] For receive antenna 9 in position II the equation described hereis produced for the form of embodiment in accordance with FIG. 1 isproduced for the receive level.

[0056] Furthermore in FIG. 2 in the receive unit 15 itself there isprovision for a further receive antenna 19 which is preferablyimplemented on a printed circuit board of the circuit of receive unit15. This receive antenna can for example feature the directionalcharacteristic shown in FIG. 4. If this receive antenna, as shown inFIG. 2, is aligned with its directional characteristic to position A,the relevant receive signal can be used to control the functionalcapabilities of the entire arrangement in evaluating the signals of thereceive antenna 9. The further antenna can however also be aligned toone of the remaining wheel positions. Overall the further receiveantenna 19 increases the redundancy of the arrangement and thereby witha suitable evaluation the security of the assignment of the wheelpositions A, B, C, D to the receive signals.

1. Arrangement to monitor at least one parameter for a number of motorvehicle wheels, comprising: a) a number of detector units, each of whichis assigned to a wheel, whereby each detector unit comprises a sensorfor recording the at least one parameter and a transmit unit with atransmit antenna, b) at least one receive antenna comprising adirectional characteristic positioned in such a way in the vehicle that,in connection with the field strengths of the signals sent by theindividual transmit units at the location of the at least one receiveantenna produces receive signals of different power in each case for atleast two of the transmit units, c) a central evaluation unit whichcomprises a receive unit, wherein the receive unit is connected to theat least one receive antenna; and d) an evaluation unit coupled with thecentral evaluation unit which compares the receive power of a receivedsignals with stored threshold values or ranges of values assigned to onewheel position in each case and assigns the received signal to a wheelposition, if the threshold value assigned to this wheel position isexceeded or the receive power lies within the range of values assignedto this wheel position.
 2. Arrangement in accordance with claim 1,further comprising means for creating the directional characteristic ofthe at least one receive antenna such that, starting from the positionof the receive antenna, they each produce sensitivities in the directionof the transmit antennas of the at least two signals to be distinguishedof the relevant transmit units, for which the difference is greater thana predefined value, in which case this value is selected to enable aclear distinction to be made between the signals using the detectedreceive power of the two signals.
 3. Arrangement in accordance withclaim 1, wherein for a vehicle with four wheels two receive antennas areprovided.
 4. Arrangement in accordance with claim 3, wherein one receiveantenna is provided in a position in the area of the front wheel and onereceive antenna is provided in a position in the area of the rear wheel,preferably in the area above the wheel arch.
 5. Arrangement inaccordance with claim 4, wherein the directional characteristic of thereceive antenna at least in the direction of the further wheel of thesame lengthwise side of the vehicle shows a low or high sensitivity andin the direction of the further front or rear wheel a high or lowsensitivity, so that the evaluation unit can use one receive antenna ineach case to undertake at least one unique assignment of the signals ofthe transmit units at the wheel positions in the direction of the lowand high sensitivity.
 6. Arrangement in accordance with claim 5, whereinthe directional characteristic of the receive antennas is created sothat the signal of the transmit unit in the area of the same wheelcreates the highest receive power in each case, which is clearlydistinguishably higher than the receive power on reception of a signalfrom the transmit unit in the direction of the high sensitivity of thedirectional characteristic.
 7. Arrangement in accordance with claim 1,wherein the transmit antennas or the entire transmit units or detectorunits are identical in design.
 8. Arrangement in accordance with claim1, wherein in the receive unit there is provision for a further receiveantenna and wherein the receive unit is positioned adjacent to atransmit unit in such a way that this receive antenna receives thesignal of the adjacent transmit unit with the greatest signal power. 9.Arrangement in accordance with claim 1, wherein the evaluation unitcomprises means for undertaking the assignment of the signals to wheelpositions on request or at regular intervals as part of an assignmentmode and when doing so assigns a characteristic identifier for thedetected wheel position transmitted by one of the transmit units in eachcase and stores it and wherein the means in normal operation assign thereceived signals to the wheel positions using a comparison between theidentifier transmitted by the transmit units and the stored assignmentinformation (identifier for wheel position).
 10. Arrangement inaccordance with claim 1, wherein the receive unit comprises acontrollable switch that in each case connects one of a number ofreceive antennas with downstream components of receive unit.
 11. Methodfor monitoring at least one parameter for a number of motor vehiclewheels, comprising the steps of: a) recording the at least oneparameter; b) transmitting the at least one parameter with a transmitantenna, positioned at least one receive antenna in such a way in thevehicle that, in connection with the field strengths of the signals sentby the individual transmit units at the location of the at least onereceive antenna produces receive signals of different power in each casefor at least two of the transmit units; and c) comparing the receivepower of a received signals with stored threshold values or ranges ofvalues assigned to one wheel position in each case and assigning thereceived signal to a wheel position, if the threshold value assigned tothis wheel position is exceeded or the receive power lies within therange of values assigned to this wheel position.
 12. The method inaccordance with claim 11, further comprising the steps of: starting fromthe position of the receive antenna, distinguishing at least two signalsof the relevant transmit units by producing sensitivities in thedirection of each of the transmit antennas, for which the difference isgreater than a predefined value, in which case this value is selected toenable a clear distinction to be made between the signals using thedetected receive power of the two signals.
 13. The method in accordancewith claim 12, further comprising the step of providing one receiveantenna in a position in the area of a front wheel and one receiveantenna in a position in the area of a rear wheel, preferably in thearea above the wheel arch.
 14. The method in accordance with claim 13,wherein the directional characteristic of the receive antenna at leastin the direction of the further wheel of the same lengthwise side of thevehicle shows a low or high sensitivity and in the direction of thefurther front or rear wheel a high or low sensitivity, so that theevaluation unit can use one receive antenna in each case to undertake atleast one unique assignment of the signals of the transmit units at thewheel positions in the direction of the low and high sensitivity. 15.The method in accordance with claim 14, further comprising the step ofcreating the directional characteristic of the receive antennas in sucha way that the signal of the transmit unit in the area of the same wheelcreates the highest receive power in each case, which is clearlydistinguishably higher than the receive power on reception of a signalfrom the transmit unit in the direction of the high sensitivity of thedirectional characteristic.
 16. The method in accordance with claim 11,further comprising the steps of: providing for a further receive antennain the receive unit and positioning the receive unit adjacent to atransmit unit in such a way that this receive antenna receives thesignal of the adjacent transmit unit with the greatest signal power. 17.The method in accordance with claim 11, wherein the evaluation unitundertakes the assignment of the signals to wheel positions on requestor at regular intervals as part of an assignment mode and when doing soassigns a characteristic identifier for the detected wheel positiontransmitted by one of the transmit units in each case and stores it andthe evaluation unit in normal operation assigns the received signals tothe wheel positions using a comparison between the identifiertransmitted by the transmit units and the stored assignment information(identifier for wheel position).
 18. The method in accordance with claim11, further comprising the step of connecting one of a number of receiveantennas with downstream components of the receive unit by means of aswitch.